Fluid control devices include various categories of equipment including control valves and regulators. Such control devices are adapted to be coupled within a fluid process control system such as chemical treatment systems, natural gas delivery systems, etc., for controlling the flow of a fluid therethrough. Each control device defines a fluid flow-path and includes a control member for adjusting a dimension of the flow-path. For example, FIG. 1 depicts a known regulator 10 including a valve body 12 and an actuator 14. The valve body 12 defines a flow-path 16 and includes a throat 18. In FIG. 1, the regulator 10 is configured in a flow-up configuration. The actuator 14 includes an upper actuator casing 20, a lower actuator casing 22, a diaphragm subassembly 30 including a diaphragm 32, and a positioning device assembly 34.
The positioning device assembly 34 includes a tubular control member 33, a coil spring 35, a central rod 36, a first spring seat 38, and a second spring seat 40. The tubular control member 33 is disposed within the upper and lower actuator casings 20, 22 and is adapted for bi-directional displacement in response to changes in pressure across the diaphragm subassembly 30. So configured, the tubular control member 33 controls the flow of fluid through the throat 18 of the valve body 12. Additionally, as is depicted, the regulator 10 includes a seat ring 26 disposed in the throat 18 of the valve body 12. When the outlet pressure of the valve body 12 is high, a sealing surface 28 of the positioning device assembly 34 may sealingly engage the seat ring 26 and close the throat 18. Similarly, absent any pressure in the actuator 14 or upon the failure of the diaphragm 32, the coil spring 35, which is carried by the central rod 36, and disposed within the tubular control member 33 biases the tubular control member 33 into an open position, which is illustrated in FIG. 1.
Still referring to FIG. 1, the coil spring 35 of the conventional regulator 10 is carried by the central rod 36 between the first spring seat 38 and the second spring seat 40. The first spring seat 38 generally comprises a flat plate that is fixedly coupled to the central rod 36. The second spring seat 40 includes a more complex structure that is fixedly coupled to an inner wall of the tubular control member 33. Typically, the second spring seat 40 is threadably coupled to the inner wall of the tubular control member 33. As depicted in FIG. 2, the second spring seat 40 comprises a one-piece member having a complex geometrical cross-section for seating and aligning the spring 35 in the tubular control member 33.
Specifically, the second spring seat 40 of the regulator 10 depicted in FIG. 2 includes a cross-sectional geometry that generally resembles a modified conical or triangular shape including a fixation portion 42, a seating portion 44, and a rod receiving portion 46. The fixation portion 42 includes a plurality of external threads 48 that threadably connect the second spring seat 40 to the tubular control member 33. The rod receiving portion 46 defines an aperture 50 for receiving the central rod 36 (as shown in FIG. 1) such that the tubular control member 33 and second spring seat 40 can move relative to the central rod 36 during operation of the regulator 10.
The seating portion 44 of the second spring seat 40 is disposed between the fixation portion 42 and the rod receiving portion 46 and is adapted to be engaged by an end of the spring 35. Specifically, the seating portion 44 includes a generally horizontal seating surface 52 and an alignment surface 54. As illustrated in FIG. 1, an end of the spring 35 seats against the seating surface 52 and an inner side of the spring 35 is disposed adjacent to and/or in contact with the alignment surface 54. So configured, the seating portion 44 of the second spring seat 40 operates to support and align the spring 35 within the tubular control member 33.
During operation, the tubular control member 33 and the second spring seat 40 move relative to the central rod 36 in response to changes in pressure across the diaphragm assembly 30. This movement causes the spring 35 to cyclically expand and compress with the movement of the tubular control member 33. However, expansion and compression of the spring 35 can result in misalignment of the first spring seat 38 relative to the tubular control member 33. This misalignment can be the result of imperfections present in the manufacturing of such springs. These imperfections can cause uneven perimeter loading of the spring 35, which can cause the spring 35 to shift laterally and contact the inner wall of the tubular control member 33 and/or push the first spring seat 38 laterally into the inner wall of the tubular control member 33. This phenomenon is generally referred to as side loading and it can cause the spring 35 and/or the first spring seat 38 to wear prematurely and/or fail. This problem is exacerbated when the spring 35 comprises a large, high rate spring that generates substantial loads.